How to Convert 285/55R20 Tire Size to Inches: A Complete Beginner-Friendly Guide

When shopping for new tires, you’ve probably come across sizes written in a strange format like 285/55R20. If you’re like most drivers, you may be wondering:
“What do these numbers mean?” or “How do I convert 285/55R20 to inches?”

You’re not alone. Many people find tire sizes confusing. But don’t worry — in this article, we’ll break it all down in simple words so you can understand exactly what your tire size means and how to convert it to inches step by step.

Whether you’re planning to upgrade your wheels, compare tire options, or just want to understand more about your vehicle, this guide will walk you through the 285/55R20 tire size conversion, tire measurements, and calculation formulas with examples.

What Does 285/55R20 Mean?

Let’s start by decoding the size 285/55R20. It may look complicated, but each part tells you something specific about the tire.

• 285 – This is the tire width in millimeters (from sidewall to sidewall)

• 55 – This is the aspect ratio or profile. It means the height of the sidewall is 55% of the tire’s width.

• R – This stands for Radial construction, which is the most common type of tire.

• 20 – This is the diameter of the rim (or wheel) in inches.

So, the tire is 285 millimeters wide, with a sidewall height that’s 55% of the width, and it fits on a 20-inch rim.

Why Convert Tire Size to Inches?

Most people in the U.S. use inches to measure things — including wheels and suspension lifts. So converting the tire size from millimeters to inches helps when:

• Comparing tire sizes

• Upgrading or modifying wheels

• Matching tires to your lift kit

• Ensuring clearance with fenders or suspension

• Buying tires online or for off-road use

Let’s break down exactly how to convert a 285/55R20 tire to inches.

Step-by-Step Conversion: 285/55R20 to Inches

Here’s how we convert this metric tire size to inches:

Step 1: Convert Tire Width (285 mm) to Inches

1 inch = 25.4 millimeters
So:

$$285÷25.4=11.22\text{ inches}$$

So the tire width is about 11.22 inches.

Step 2: Calculate Sidewall Height

Now, we use the aspect ratio (55%) to find the sidewall height.

$$285\times 0.55=156.75\text{ mm}$$

Convert to inches:

$$156.75÷25.4=6.17\text{ inches}$$

So each sidewall is 6.17 inches tall.

Step 3: Calculate Total Tire Diameter

The full diameter includes two sidewalls + the rim:

$$6.17(sidewall)\times 2+20(rim)=32.34\text{ inches}$$

So, a 285/55R20 tire has an overall diameter of about 32.34 inches.

Step 4: Calculate Tire Circumference

If you want to calculate how far your tire travels in one full rotation, use the formula:

$$\text{Circumference}=\pi \times \text{Diameter}$$ $$\pi \times 32.34\approx 101.58\text{ inches}$$

So the tire’s circumference is about 101.6 inches.

Step 5: Calculate Sidewall Height in Inches (Individually)

For lifted trucks or off-road builds, knowing just the sidewall can help with clearance.

• Sidewall height: ~6.17 inches

• Rim size: 20 inches

• Total height: 32.34 inches

Quick Summary Table: 285/55R20 in Inches

Why Does Tire Size Matter?

Understanding your tire size isn’t just for looks. It affects your:

• Speedometer accuracy

• Fuel economy

• Ride comfort

• Traction

• Clearance

• Towing capacity

If you install tires that are too big or too small without recalibrating, your speedometer will be off, and your car may not handle properly.

Comparing 285/55R20 with Other Sizes

Wondering how 285/55R20 stacks up against similar sizes?

Here’s a quick comparison:

If you’re changing tire size, make sure the difference is within 3% of the original diameter to avoid affecting your vehicle’s computer systems and gear ratios.

Tools to Help: Tire Size Calculators

If math isn’t your thing, there are free tire size calculators online. Just enter your current size and new size, and it will show:

• Side-by-side comparison

• Height difference

• Speedometer impact

• Clearance impact

Some popular ones include:

• tiresize.com

• discounttire.com

• 1010tires.com

Frequently Asked Questions (FAQ)

Q: Is 285/55R20 a common truck tire size?

A: Yes, it’s popular for full-size trucks like the Ford F-150, Chevy Silverado, GMC Sierra, and Ram 1500.

Q: Can I fit 285/55R20 tires on a stock suspension?

A: Most trucks can fit this size with little or no modification, but check for fender clearance.

Q: Will 285/55R20 affect my fuel economy?

A: Slightly. Larger tires can reduce fuel efficiency due to extra weight and rolling resistance.

Q: How do I know if a tire will fit my rim?

A: The last number (20) tells you the rim diameter. So, a 285/55R20 tire fits a 20-inch rim.

Q: What’s the biggest tire I can run on my truck?

A: It depends on your make/model and whether you have a lift kit. Consult your manual or a tire professional.

Tire Size Tips for DIYers and Shoppers

• Always double-check with your vehicle’s manual or dealer before upgrading tire sizes.

• A small change in tire height can affect your gear ratio and speedometer.

• If you go taller or wider, consider whether you need a suspension lift or leveling kit.

• Keep in mind the load index and speed rating of new tires.

• Why Choose a 285/55R20 Tire?

Here are a few benefits of this size:

• Great balance of width and ride comfort

• Fits stock rims on many trucks

• Looks aggressive without being too big

• Good traction for on-road and light off-road use

• Available from top tire brands like BFGoodrich, Nitto, Michelin, Toyo, and Goodyear

Final Thoughts: 285/55R20 Tire Size Conversion in Inches

Now you know that a 285/55R20 tire has:

• Width: ~11.22 inches

• Sidewall height: ~6.17 inches

• Overall height: ~32.34 inches

• Fits a 20-inch rim

Understanding these numbers helps you make smarter choices for your vehicle — whether you’re shopping for tires, upgrading wheels, or planning a lift.

Quick Recap:

• Tire size 285/55R20 means 285 mm wide, 55% sidewall, and 20-inch rim

• Convert to inches by dividing millimeters by 25.4

• Tire width = 11.22 inches

• Sidewall height = 6.17 inches

• Overall diameter = 32.34 inches

• Use tire calculators for quick comparisons

• Always verify fitment for your vehicle

• Also, visit Home Design Looks for more quality information.